The present invention relates to the art of earth boring and, more particularly, to a drill string unit adapted to be connected as a member of a rotary drill string. A drill string unit generally comprises a body threaded at at least one end for incorporation in a rotary drill string. This body supports a multiplicity of bearing shafts. A rotary cutter is rotatably mounted upon each bearing shaft. The drill string unit must operate under very severe earth boring conditions, and the size and geometry of the drill string unit is restricted by the operating characteristics. The economics of petroleum production and earth boring demand a longer lifetime and improved performance from the drill string unit. Attempts to increase the lifetime and improve the performance of the drill string unit have included sealing and lubricating the rotary cutters. The sealing and lubricating systems have encountered problems created by the environmental conditions encountered in earth boring.
A wide variety of environmental conditions affect the performance of the drill string unit. For example, the temperature will rise as the borehole penetrates deeper into the earth and temperatures of up to 325.degree. F. are presently being encountered. Deep wells now being drilled are expected to result in environmental temperatures of up to 400.degree. F. and the drilling of steam wells results in environmental temperatures as high as 550.degree. F. The elevated temperatures have an adverse effect on the lubricant, the structural elements of the lubrication system, the structural elements of the seal members and a substantial increase in the pressure of the lubricant within the lubrication system may be encountered.